1. Field of the Invention
The present invention relates to motorcycle frames, particularly rear forks for such frames, and specifically to a rear swinging fork for a motorcycle, fabricated as a one-piece die cast component and featuring two longitudinal supporting arms each with three supporting walls and a transverse cross member connecting the supporting arms.
2. Background Art
The main function of the rear swinging fork is to connect the rear wheel of the motorcycle to the frame and to direct spring damper forces into the frame.
In the art, a distinction is made between so-called single and double swing arm types, depending on the number of swing arms of the rear swinging fork supporting the rear wheel. Negotiating a bumpy road surface subjects the rear swinging fork to heavy flexural loading, due to the vertical forces stressing the rear swinging fork when the wheel hits the road again after leaping. The bending moment on the fork is particularly high in the region of the strut pivot link. The rear wheel of the motorcycle builds up side forces which stress the fork with forces on cornering, resulting in the footprint of the tire being displaced to a torsional moment stressing the rear swinging fork. In other words, the rear swinging fork is a highly stressed component of the motorcycle, and although needing to be engineered to be stiff, since as a vertically oscillating mass it influences the center of gravity location of the motorcycle and thus the response, it nevertheless must be a lightweight component.
Rear swinging forks for motorcycles having twin supporting arms are already known in a wealth of different variants. Thus, for example, they are known as welded structures with steel tubes, but also as so-called built-up forks assembled from a plurality of components produced by casting, forging, extrusion or made of sheet metal. Also known are rear swinging forks cast in one piece, involving hitherto mainly sand castings but also permanent moldings.
EP 0295 799 B1 discloses a rear swinging fork as a die-cast aluminum component. This known rear swinging fork has, extending in its longitudinal direction, twin supporting arms and twin cross members connecting the supporting arms in the region of a strut mount of the motorcycle and in the region of both mounts for pivotally mounting the rear swinging fork to the frame. In other words, there are two fulcrum mounts of the fork. The two supporting arms of this known type of rear swinging fork are each engineered so that a cross-sectional view through the two supporting arms has the shape of an inverted “U.” Stated differently, this device has two side supporting walls which, when the known rear swinging fork is mounted in the frame, are downswept in the direction of a mount at rear swinging fork for strut; it also has an upper supporting wall connecting the two side supporting walls and arranged opposite the strut mount, resulting in a configuration of side supporting walls and upper supporting wall each for each supporting arm in the form of an inverted U.
Situated within the cavity between the two side supporting walls and the upper supporting wall of this known rear swinging fork is a stiffening rib. The rib is oriented in the longitudinal direction of the rear swinging fork from the two fulcrums of the fork mount of the cross member connecting each of the supporting arms up to the rear wheel mount of the motorcycle. The section of this known rear swinging fork between the cross member in the region of the two fulcrums of the fork mount of the supporting arms and the further cross member is engineered with an opening forming an inherently closed space between the further cross member (located displaced to the rear in the longitudinal direction of the rear swinging fork extending from the fork mount fulcrum in the direction of the wheel mount) and the cross member between the two fork mount fulcrums.
When a motorcycle featuring this known rear swinging fork is exposed to highly dynamic performance, such as negotiating bumpy road surfaces or rough terrain with subsequent landing impact of the motorcycle—as with off-road competition models or also other dynamic models—or when, for instance, negotiating bumpy roads with a motorcycle equipped with such known rear swinging forks, there results in the rear swinging fork heavy flexural stresses. Jouncing of the strut or spring damper subjects this known fork to high tensile stress in the region of the underside of the supporting arms facing the road surface to high tensile stress. Consequently, the fork is required to absorb maximum tensile stress at the cross-section of the supporting arms having a minimal material wall thickness. This poses the risk of stress cracks occurring particularly in the lower region of the supporting arms, ultimately resulting in fracture of the rear swinging fork.
Against the foregoing background, the present invention was developed with the object of sophisticating the rear swinging fork so that the cited disadvantage of stress cracking, in the region of portions of the rear swinging fork exposed to high tensile stress and the risk of buckling the upper supporting wall of the supporting arms, is avoided whilst defining a configuration of the rear swinging fork which combines torsional rigidity with low weight.